Process for the polymerization and copolymerization of ethylene, using a gas injection device

Abstract

Process and device for cooling a mixture of polymer and monomer from a reactor in which ethylene is polymerized or copolymerized at a pressure greater than 1,000 bars between a pressure-reduction valve located downstream of the reactor and a medium pressure separator. The cooling is achieved by injecting monomer at a pressure below that of the separator using a device with a nozzle cross-section S 1 through which the mixture from the reactor passes, a convergent mixing zone in which the mixture is mixed with the monomer injected, which is supplied at a flow rate q, and a diffuser of throat cross-section S 3 , which makes it possible to bring the mixture to the pressure of the separator. The dimensions of the device are such that the ratio Q/S 1 is between 0.20 and 1.35 t/hr.mm2 and the ratio (q+Q/S 3 is between 0.1 and 0.3 t/hr.mm2.

We claim: 
     
       1. In a process for the polymerisation or copolymerisation of ethylene in a reactor at a pressure greater than 1,000 bars, which process includes cooling the reactor effluent comprising a mixture of polymer and monomer between a pressure-reduction valve located downstream from the reactor and a separator operating at 200 to 500 bars, separating polymer from monomer in the separator and recycling separated monomer to the reactor, the improvement comprising: (A) injecting fresh monomer into the reactor effluent between the valve and the separator at a temperature lower than that of the reactor effluent and at a pressure below that of the separator using a gas injection device comprising at least one assembly comprising (1) a convergent ejection nozzle of throat cross-section S 1  substantially at the downstream end of said nozzle, through which arrives at a flow rate Q, motive fluid consisting of reactor effluent; (2) a convergent mixing zone which said fluid enters after passing through said ejection nozzle to be mixed with the monomer injected, which is supplied at a flow rate q, said mixing zone at its upstream end having a larger cross-section than S 1  ; and (3) a diffuser of throat cross-section S 3  constituting the downstream end of said mixing zone, wherein the ratio Q/S 1  is between 0.20 and 1.35 t/hr.mm 2  and the ratio (q+Q)/S 3  is between 0.1 and 0.3 t/hr.mm 2 , the pressure of said fluid entering said ejection nozzle being higher than the pressure of said separator and between about 400 and 2,500 bars and the pressure of said injected monomer being not greater than 180 bars whereby the pressure of the fluid is brought down to the pressure of the separator; and   (B) substantially all of said fresh monomer for the polymerization process being supplied by its injection into the reactor effluent using said gas injection device.   
     
     
       2. The process according to claim 1 for the polymerization of ethylene wherein the fresh monomer injected is fresh ethylene under a pressure of about 50 bars. 
     
     
       3. Process according to claim 1, wherein the reactor is of the autoclave type. 
     
     
       4. Process according to claim 1, wherein the reactor is of the tubular type. 
     
     
       5. The process of claim 1, wherein the ratio (q+Q)/S 3  is between 0.16 and 0.24 t/hr.mm 2 . 
     
     
       6. The process of claim 1, wherein the ejection nozzle is provided downstream from the throat of cross-section S 1  with a diverging section, extending from said throat to an outlet of cross-section S' 1 , the ratio S' 1  /S 1  being between 1 and 1.5, and S' 1  being less than the cross-section of the upstream end of said mixing zone. 
     
     
       7. The process of claim 1, wherein all of said fresh monomer is supplied by injection using said gas injection device. 
     
     
       8. The process of claim 1, wherein the fresh monomer is injected under a pressure of about 50 bars. 
     
     
       9. The process of claim 1, wherein the ratio Q/S 1  is between 0.39 and 1.35 t/hr.mm 2  and the pressure of the fluid entering said ejection nozzle is between about 600 and 2500 bars. 
     
     
       10. The process of claim 5, wherein the ratio Q/S 1  is between 0.39 and 1.35 t/hr.mm 2  and the pressure of the fluid entering said ejection nozzle is between about 600 and 2500 bars.